Between Elemental Match and Mismatch: From K12Ge3.5Sb6 to Salts of (Ge2Sb2)2−, (Ge4Sb12)4−, and (Ge4Sb14)4−

Abstract The solid mixture “K2GeSb” was shown to comprise single‐crystalline K12Ge3.5Sb6 (1), a double salt of K5[GeSb3] with carbonate‐like [GeSb3]5− anions, and the metallic Zintl phase K2Ge1.5. Extraction of 1 with ethane‐1,2‐diamine in the presence of crypt‐222 afforded [K(crypt‐222)]+ salts of several novel binary Zintl anions: (Ge2Sb2)2− (in 2), (Ge4Sb12)4− (in 3), and in the presence of [AuMePPh3] also (Ge4Sb14)4− (in 4). The anion in 2 represents a predicted, yet heretofore missing pseudo‐tetrahedral anion. 4 comprises a cluster analogous to (Ge4Bi14)4− and (Ga2Bi16)4−, and thus one of the most Sb‐rich binary p‐block anions. The unprecedented cluster topology in 3 can be viewed as a defect‐version of the one in 4 upon following a “dead end” of cluster growth. The findings indicate that Ge and Sb atoms are at the border of a well‐matching and a mismatch elemental combination. We discuss the syntheses, the geometric structures, and the electronic structures of the new compounds.


Synthesis of compounds
1.2.1 Synthesis of K 12 Ge 3.5 Sb 6 (1) K 12 Ge 3.5 Sb 6 was prepared by fusion of the elements (ratio: 2:1:1) at 950 °C for 48 hours in a niobium tube, sealed within an evacuated silica ampoule. The single-crystalline solid formed alongside some elemental Ge.
The resulting intense orange solution was filtered through a standard glass frit. Because carefully layering with toluene (15 mL) and storing for crystallization was not successful, the solvent was evaporated until first crystals were visible. The Schlenk tube was then stored for further crystallization at 5 °C. After 1 day, crystals of compound 2 (orange sticks; >85 % of the crystals) and 3 (dark brown ellipsoids; >5 % of the crystals) formed in the Schlenk tube in approx. 70 % yield overall.
After 40 days, block-like metallic black crystals of compound 4 formed at the wall of the Schlenk tube in approx. 20 % yield.

S4
Powder X-ray diffraction (PXRD) of K 12 Ge 3.5 Sb 6 (1) Powder X-ray diffraction (PXRD) data were collected on a Stoe StadiMP diffractometer system equipped with a Mythen 1 K silicon strip detector and Cu-K α -radiation (λ = 1.54056 Å). The sample of 1 was filled into a glass capillary (0.3 mm diameter), which was sealed air-tightly with soft wax. The tube was then mounted onto the goniometer head using wax (horizontal setup) and rotated throughout the measurement. The diffraction pattern is shown in Figure S1. we thus assume that the additional reflections stem from at least one so far unidentified minor binary or ternary by-product that seems to be tightly attached to the single crystals we investigated.

General considerations
The data for the X-ray structural analyses were collected at T = 100.0 K with Mo-Kα-radiation (λ = 0.71073 Å) on area detector systems Stoe IPDS/2T for 1, 2 and 4, Cu-Kα-radiation (λ = 1.54186 Å) on an area detector system Stoe StadiVari and T = 150.0 K with Ga-Kαradiation (λ = 1.34143 Å) on a Stoe StadiVari diffractometer for 3. The structures were solved by dual space methods of SHELXT from SHELXL-2018/136, [2] and refined by full matrix leastsquares methods against F 2 with the SHELXL program. [3] All hydrogen atoms were kept riding on calculated positions with isotropic displacement parameters U = 1.2 Ueq of the bonding partners. The crystal quality of compound 4 was comparably poor. Zintl compounds, especially ones with highly symmetric clusters in them, tend to suffer from some inherent rotational disorder of the clusters, which can result in diffraction data such as the ones observed here.
As we could not reproduce this compound so far, we could not collect a better data set.
However, we were able to unambiguously determine the crystal structure from it.
Crystallographic data for the structures of 1 -4 (Table S1 - Figure S6. Molecular structure of the (Ge 4 Sb 14 ) 4− anion in 4 with full labelling scheme.

Micro-X-ray fluorescence spectroscopy (µ-XFS) analysis of 1, 2 and 3
All µ-XFS measurements were performed on single crystal samples with a Bruker M4 Tornado, equipped with an Rh-target X-ray tube, poly capillary optics and a Si drift detector. The emitted fluorescence photons are detected with an acquisition time of 180 s. Quantification of the elements is achieved through deconvolution of the spectra. Results are summarized in Table   S7. Figures S8-S10 show the spectra for 1 -3 along with the results of the deconvolution algorithm. Several measurements produced unreasonably large values for the % K. Removal of K from the calculations afforded excellent agreement with the expected atomic ratio of close to Ge 2.00 Sb 2.00 in 2, and Ge 4.00 Sb 12.00 , respectively. It is a typical finding though for such compounds, the reasons for which could not be clarified so far. Measurements of compound 4 were not possible owing to the failure to reproduce it to date.  Figure S8. Micro X-ray fluorescence spectrum of 1 with the results of the deconvolution algorithm. Figure S9. Micro X-ray fluorescence spectrum of 2 with the results of the deconvolution algorithm. S18 Figure S10. Micro X-ray fluorescence spectrum of compound 3 with the results of the deconvolution algorithm.

Electrospray ionization mass spectrometry (ESI-MS)
investigations of the extraction process of

Methods
In order to explore the possible formation pathway, we prepared a series of reactive extraction solutions of K 12 Ge 3.5 Sb 6 (1) in en, like those from which compounds 2 and 3 were crystallized.
The reaction solutions were allowed to stir for 5 minutes, 30 minutes, 3 hours, 1 day, 1 week and 2 weeks, before the measurements.
All mass spectra were recorded with a Thermo Fischer Scientific Finnigan LTQ-FT spectrometer in negative ion mode. The solutions were injected into the spectrometer with gastight 250 µL Hamilton syringes by syringe pump infusion. All capillaries within the system were washed with dry en/toluene mixture (1:1) 2 hours before and at least 10 min in between measurements to avoid decomposition reactions and consequent clogging.
The following ESI parameters were used: Spray Voltage     Structural optimization based on the crystallographic data was conducted using the Vienna Ab initio Simulation Package (VASP) [5] with a 1×1×2 supercell to model the unusual distribution of the sub-occupied Ge2 and Ge3 atoms along the c-axis. Subsequent bond analysis was conducted with the help of the Local Orbital Basis Suite Towards Electronic Structure Reconstruction (LOBSTER) [6] package by projecting the PAW-based [7] wavefunctions onto a local orbital basis. This allowed for the calculation of Löwdin charges [8] and bond orders as expressed in the crystal orbital bond index (COBI). [9]

Methods of the DFT Studies of Molecular Anions
Simultaneous optimization of the geometric and electronic structures of the anions in 1 -4 were undertaken using the program system Turbomole V7.5.1. [10][11] Density functional theory (DFT) [12] methods were employed throughout these studies. We employed the TPSS functional [13] and basis sets of quality dhf-TZVP [14] with additional use of auxiliary basis sets [15] and effective core potentials [16] at the Sb atoms. Negative charges were compensated with the conductor-like screening model (COSMO). [17] Partial charges were calculated by means of Mulliken [18] and natural population analyses (NPA). [19] Localized molecular orbitals were obtained according to Boys' method. [20] The anions in 1 -4 were initially optimized without any symmetry restrictions and later reoptimized with higher symmetry point groups, where appropriate (1: D 3h , 2: C 2v , 4: C 2h ).
Calculation of force constants with the module NumForce verified all structures to be minima on the potential energy hypersurface.

Details of the Quantum Chemical Investigations of Compound 1
The chosen convergence criteria for the VASP structure optimization were 10 −8 eV for electronic and 5 • 10 −3 eV/Å for ionic iteration steps whereas the kinetic energy cutoff was 500 eV. The VASP-recommended PAW pseudopotentials were chosen for these calculations using a k-point mesh of 0.02 to 0.04 Å −1 with Blöchl's tetrahedron integration method [21] and a smearing factor of 0.05 eV. In addition, the solid-state-optimized generalized gradient approximation (GGA) [22] and the Becke-Johnson dampened D3-Method to approximate the van-der-Waals interactions [23] were chosen.

Figure S26
. Localized molecular orbitals (LMOs) of the most stable isomer of the anion in 3 (isomer I in Figure S25), representing different two-center (a-v) and one-center (w-ak) interactions: Ge-Ge bonds (a-c), Ge-Sb bonds (d-k), Sb-Sb bonds (l-v), lone-pairs at a Ge atom (w, x) and at Sb atoms (y-ak). Contour values are drawn at ±0.05 a.u. Figure S27. Canonical molecular orbitals of the most stable isomer of the anion in 3 (isomer I in Figure S25). Contour values are drawn at ±0.03 a.u. Figure S28. Representative localized molecular orbitals (LMOs) of the most stable isomer of the anion in 4, representing different two-center (a-h) and one-center (i, j) interactions: Ge-Ge bonds (a-b), Ge-Sb bonds (c-d), Sb-Sb bonds (e-h), and of the lone-pair at Ge atoms (i) and at µ-bridging Sb atoms (j). Contour values are drawn at ±0.05 a.u. Figure S29. Canonical molecular orbitals of the most stable isomer of the anion in compound 4. Contour values are drawn at ±0.03 a.u. Figure S30. Calculated isomers of (Ge 4 P 14 ) 4− and relative energies ΔE with respect to the global minimum structure. Figure S31. Calculated isomers of (Ge 4 As 14 ) 4− and relative energies ΔE with respect to the global minimum structure. Figure S32. Calculated isomers of (Ge 4 Sb 14 ) 4− (anion in 4) and relative energies ΔE with respect to the global minimum structure. Figure S33. Calculated isomers of (Ge 4 Bi 14 ) 4− and relative energies ΔE with respect to the global minimum structure.